Method of making shell moulded articles therefor

ABSTRACT

Method of making shell-moulded articles comprising passing a fluidizing gas through a bed of refractory granular material to fluidize the latter, effecting a reduction, relative to the permeability of the fluidized bed, of the permeability of the shell mould to the fluidizing gas to a value sufficiently less than the permeability of the fluidized bed to substantially ensure that the gas does not pass through the mould in preference to the bed, immersing the mould in the bed to a depth such that, on collapse of the fluidized bed, the mould is supported by the refractory material with the downgate of the mould clear of the surface of the material, collapsing the bed by ceasing to pass fluidizing gas through it, pouring the mould, restoring the passage of fluidizing gas through the refractory material to fluidize the latter, and removing the poured mould from the bed.

United States'Patent [1 1 Fallows et a1.

METHOD OF MAKING SHELL-MOULDED ARTICLES THEREFOR Inventors: John Fallows, Turbridge Wells,

Kent; John Edward Worthington, Tonbridge, Kent, both of England Polygram Casting Co., Limited, Kent, England Filed: Nov. 30, 1970 Appl. No.: 93,699

Assignee:

References Cited UNITED STATES PATENTS 5/1961 Carter 164/361 X 11/1952 Traenkner 164/72 X 9/1971 Chapman et al 164/47 X June 26, 1973 Primary Examiner-J. Spencer Overholser Assistant Examiner-David S. Safran Attorney-Holcombe, Wetherill 8L Brisebois [5 7 ABSTRACT Method of making shell-moulded articles comprising passing a fluidizing. gas through a bed of refractory granular material to fluidize the latter, effecting a reduction, relative to the permeability of the fluidized bed, of the permeability of the shell mould to the fluidizing gas to a value sufficiently less than the permeability of the fluidized bed to substantially ensure that the gas does not pass through the mould in preference to the bed, immersing the mould in the bed to a depth such that, on collapse of the fluidized bed, the mould is supported by the refractory material with the downgate of the mould clear of the surface of the material, collapsing the bed by ceasing to pass fluidizing gas through it, pouring the mould, restoring the passage of fluidizing gas through the refractory material to fluidize the latter, and removing the poured mould from the bed.

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same 0F 2 COOL ING PUUR/NG RUN WA Y INVENTOR J. FALLOWS BY 3'. E .WoR-m n4 QTON ATTORNEYS METHOD OF MAKING SHELL-MOULDED ARTICLES THEREFOR In some cases it is necessary to provide support for shell moulds while the molten metal is being poured into them in order to prevent breaking or distortion of the thin shell. This necessity increases as the size and weight of the casting to be produced increases.

It is known to provide such support by placing the shell mould in a container and then pouring into the container granular refractory material, for example steel shot, grit, gravel, green sand, compacted if necessary, to support the mould. After the casting has been poured and has solidified, it can be removed from the container. The granular support material must then be removed from the container in order that the process can be repeated in pouring another casting. Sometimes the container is made with an openable bottom in order to facilitate the removal of the granular material.

This process involves handling large quantities of granular material and it is wasteful of time, labor and handling equipment.

The present invention provides an improved method of and apparatus for making shell-moulded articles.

According to the present invention a method of making shell-moulded articles comprises the steps of first passing a fluidizing gas through a bed of refractory material to fluidize the latter, then effecting a temporary reduction, relative to the permeability of the fluidized bed, of the permeability of the shell mould to the gas used to fluidize the bed to a value sufficiently greater than the permeability of the fluidized bed to ensure that the gas does not pass in any substantial amount through the mould in preference to through the fluidized bed, after which the shell mould is immersed in the fluidized bed to a depth such that, on collapse of the bed, the mould is supported in the bed with the or each downgate of the mould clear of the surface of the material, then collapsing the bed by ceasing to pass fluidizing gas through it, pouring the mould, then restoring the passage of fluidizing air through the refractory material to fluidize the latter and finally removing the poured mould from the bed.

In one embodiment of the invention, the permeability of the mould is reduced by enveloping the mould with a substance impervious to the fluidizing gas, the substance being destroyed at least in part during pouring so as to maintain at an acceptable value the permeability of the mould to gases evolved during pouring.

The mould may be wrapped in a sheet of the substance or the mould may be placed in a bag or container of that substance. The sheet, bag or container must either not close the or each downgate of the mould or if the or each downgate is closed, it must be opened to permit pouring. Opening may be effected simply by pouring molten metal to be cast on to that part of the substance covering a downgate and using the heat of the metal to destroy that part of the substance. The sheet, bag or container may be of a plastics material, for example polythene.

Alternatively, the mould may be coated, for example by spraying, with a suitable substance which reduces the permeability of the mould to fluidizing gas and is destroyed during pouring or with another substance which, whilst reducing the permeability of the mould to fluidizing gas down not reduce to an unacceptable level the permeability of the mould to gases emitted during pouring. An example of the first-mentioned substance is paraffin wax and bentonite is an example of the other substance.

In the case of moulds of small internal capacity, the required reduction of permeability to fluidizing gas may be achieved by closing the or each downgate of the mould and then opening it or them prior to pouring. A removable stopper may be used to close a downgate.

The required relationship between the permeability of the shell mould and that of the fluidized bed may be achieved by utilizing, as the refractory material, a granular substance whose grain size is such that, when fluidized, the permeability of the fluidized bed is sufficiently greater than that of the shell mould to ensure a requisite degree of fluidization is retained in the immediate vicinity of the shell mould after the latter is immersed in the fluidized bed.

The thermal conductivity of the fluidized bed and thus the rate of cooling the mould during pouring may be increased by using material having a higher thermal conductivity than the conventional refractory material. Where the conventional refractory material is sand, the material may be zircon sand, chromite or olivine.

The refractory material comprising the bed may be dry sand or other suitable refractory material, the material having a permeability for gases emitted during pouring which is greatly in excess of the shell mould at this stage in the process.

Each shell mould may be supported in an open-mesh basket during movement into and out of the bed and during pouring. Larger shell moulds may have individual baskets but it may be possible to pack several smaller moulds into one basket.

In a particular method embodying the invention, the bed is compacted prior to pouring by vibrating the container. The latter may be vibrated during the time that the bed is fluidized so that when the fluidization is stopped the bed is rapidly compacted. Vibration may cease shortly after fluidization is stopped.

By way of example only, a method embodying the invention and apparatus for carrying out the method will now be described in greater detail with reference to the accompanying drawings of which:

FIG. 1 is a side view'partly in section of part of the apparatus,

FIG. 2 is a plan view of the apparatus shown in FIG. 1, and

FIG. 3 shows in diagrammatic form only, a plan view of part of another form of apparatus.

FIGS. 1 and 2 show a container 1 holding granular refractory material 2 which is supported on a fluidizing tile 3 below which the container is adapted to form an air box 4 connected via ducting 5 containing a flow control valve 6 to a source (not shown) of fluidizing air. The tile 3 acts, in known manner, to ensure an even flow of fluidizing air throughout the material 2 thereby effectively fluidizing the latter.

Shell moulds which have been manufactured by any one of the conventional techniques are supported in an open-framework holder 7. The precise form of the holder 7 will, of course, depend upon the configuration and size of the moulds but a typical holder is shown in the drawings. It has side members 8 from whose lower ends extend transverse supports 9 whose ends are joined by longitudinal frame members 10 from which extend mould supports 11. As can be seen from FIG. 2, the mould supports are channel sections and are arranged in pairs with the open sections of a pair facing one another. The transverse supports 9 and the frame members 10 also support a wire mesh floor 12 whose purpose will be described later. The shell moulds, which normally have a peripheral flange, are located between the mould supports 11 with the mould flanges engaged in the channel section supports 11.

Prior to being placed in the framework 7, the shell moulds which, in FIGS. 1 and 2, are shown diagrammatically at 13 are placed in bags 14 of polythene, the mouths of the bags being uppermost and left open to allow access to the downgates 15 of the shell moulds. Once the shell moulds have been placed in the framework 7 with their flanges engaged in the supports 11 as described above, a locking bar 16 is passed through apertures in the side members 8 to prevent upward movement of shell moulds which might otherwise take place.

The procedure to be followed when using the apparatus is as follows:

The container is loaded with a suitable granular material, for example silicon sand and the ducting 5 connected to a source of fluidizing air. Shell moulds to be poured are placed in polythene bags as described above and then placed in the holder 7. Fluidizing air is then admitted to the air box 4 by opening valve 6 and when the granular material 2 is fully fluidized the holder is lowered into the fluidized material until brackets 17 on the side members 8 rest on the upper edge of the container.

The depth to which the shell moulds are immersed in the fluidized material leaves the downgates 15 well clear of the surface of the material and is such that when the valve 6 is closed to collapse the fluidized bed of material 2, the shell moulds are firmly supported in the material.

After the bed has been collapsed, the material may be compacted by vibrating the container 1. This improves the support afforded to the shell moulds by the granular material. The container 1 might, for example, be mounted on an electrically operated compacting table which is shown in FIG. 1 as a dotted-line rectangle 18.

The shell moulds are now ready to be poured and this is carried out in the conventional manner. The heat of the molten metal will melt and burn off the greater part of the polythene bags so that gases evolved during pouring can pass through the walls of the shell mould.

After a short period for cooling, fluidizing air is readmitted to the air box to fluidize the material 2 after which the holder 7 can readily be lifted from the container. If, during cooling, there has been any disintegration of the shell moulds, the wire mesh floor 12 will retain the pieces as the holder is lifted from the container thereby avoiding deleterious contamination of the material 2. The process can then be repeated with fresh shell moulds.

It will be appreciated that the shell moulds are firmly supported during pouring and this enables a reduction to be made in the wall thickness of the moulds. Such reduction results in short investment and cure time during mould production as well as requiring a smaller volume of shell moulding material.

Upon using the fluidized bed described above to facilitate the pouring of a number of moulds in succession, it will be found that the temperature of the material forming the bed will rise. Although the temperature of the material may rise to as high as 180 C, this does not adversely affect the protection afforded by the polythene bag during the time the moulds are immersed in the bed before pouring starts. There is, of course, a greater increase in temperature of the granular material as a mould is poured and this destroys the polythene bag. However, by the time the poured metal has solidified sufficiently and a succeeding fresh mould has been immersed, the temperature of the granular material has fallen to the value indicated above.

The method described above can readily be introduced into a foundry as an automated production line. Suitable apparatus may take one of a number of different forms. There may, for example, be a number of containers similar to container 1 described above arranged in a line beneath an overhead hoise adapted to lower a holder full of shell moulds into a container and, later, lift the holder from the container after the moulds have been poured. In the line of containers, one or more will be waiting to receive holders with empty shell moulds, in another or others, metal in poured shell moulds will be undergoing solidification whilst there may be yet another or others from which the holders are being removed, the poured metal having solidified sufficiently to permit removal from the supporting bed of material.

Alternatively, several containers are mounted upon a turntable which indexes each through a series of stations in a first one of which a holder with empty shell moulds is loaded into a container the support material in which has been fluidized. in a second station, fluidization is stopped and the moulds poured after which the container passes through a cooling station to a holder removal station when the holder is taken out and transferred to a place where the cast objects are removed from the moulds. Apparatus of the form just described is illustrated in more detail in FIG. 3.

Mounted upon a turntable 19 is a series of containers 20 each of which is generally similar to the container shown in FIGS. 1 and 2. The ductings 21 of the containers 20 are all joined to a central plenum chamber 22 containing a fan driven by an electric motor (not shown) to provide a source of fluidizing air of high flow rate but relatively low pressure. Automatically operated flow control valves 23 control the admission of fluidizing air to the air boxes of the containers. The valves 23 may, for example, be cam-operated as the turntable rotates to move the containers through the series of stations.

Shell mould holders are conveyed to the turntable via a conveyor indicated diagrammatically at 24. The conveyor may comprise an overhead track which brings loaded shell mould holders from a loading area (not shown) to a container in position I and then lowers the holder into the container. When in position I, valve 22 of the container has been opened and the granular material therein is fluidized. When the holder is in position in the container, the turntable rotates and brings the container to position II at which the valve 22 is closed. Fluidization ceases and, if necessary, the granular material is compacted by vibrating the container. Meanwhile, a subsequent container has moved into position I and shell mould holder has been loaded into the container. The turntable then resumes rotation bringing the first-mentioned container into position III. In that position, the container is adjacent a runway or runways for transferring molten metal from a crucible (not shown) to the shell moulds. Pouring of the moulds commences in position III and may continue to position IV or beyond according to the capacity of the shell moulds.

Further rotation of the turntable advances the container in stages to position V, where valve 22 is openedto allow fluidizing air to enter the air box of the container and so fluidize the granular material therein to permit removal of the shell mould holder. Removal is effected by a conveyor, indicated diagrammatically at 25, similar to conveyor 24 referred to above. The holder is transported by conveyor 25 to an area where the shell moulds are stripped away to release the cast objects. The container moves on, in due course, to position V] where the valve 22 is shut and fluidization of the granular material in the tank ceases.

The path between positions IV and V represents a cooling zone during which the molten metal is solidifying. The polythene bags are burnt-off during movement of each container along this path to permit egress through the mould walls of gases evolved during pouring and subsequently.

It is preferable to vibrate the container at station 1 because vibration tends to make the fluidization more uniform. Also, when fluidization is stopped, the maintained vibration of the container ensures that the bed collapses and compacts rapidly. Vibration is stopped within a short time, for example 2 to 4 seconds after fluidization ceases.

As has been explained above, the holder for the shell moulds may take any one of a number of different forms depending upon the configuration, size and number of the moulds it is to receive. In addition, other methods of supporting the holder in the container may also be used. The depth to which the holder is lowered into the container must be controlled so that the granular material in the bed cannot enter the downgates of the shell moulds either during immersion or after the fluidization is stopped prior to pouring the moulds.

A grid or a series of cross bars may be fixed across the container adjacent the bottom thereof care being taken to ensure that there is no deleterious effect on the fluidization of the bed. The holder is then lowered into the container until it rests on the grid or cross bars.

Such a construction is not suitable for use with moulds whose downgates are not all of the same height above the bottom of the holder on which they rest.

An alternative form of holder would then be used which has hooks or projections to engage the top edge of the container and which are adjustable to control the depth to which the holder can be lowered into the container. in another form of holder, hooks or projections are provided to support the moulds at positions which ensure that their downgates are all at the same height above the bottom of the holder.

Further, it will be understood that it is not essential that the mesh floor described above forms an integral part of the holder. The mesh floor could be a separate component with handles or other means for periodi- Cally withdrawing the mesh from the container in order to iemove debris and other contamination which may have collected in the granular material in the container. It may not be necessary to withdraw the mesh floor after each poured set of moulds has been removed from the container.

We claim:

1. A method of making a shell-moulded article comprising the steps of first passing a fluidizing gas through a bed of refractory granular material to fluidize the latter, then effecting a temporary reduction, relative to the permeability of the fluidized bed, of the permeability of a shell mould having a downgate to the fluidizing gas to a value sufficiently less than the permeability of the fluidized bed to ensure that the gas does not pass, in any substantial volume, through the mould in preference to the bed, after which the mould is immersed in the bed to a depth such that, on collapse of the fluidized bed, the mould is supported by the refractory material with said downgate clear of the surface of the material, then collapsing the bed by ceasing to pass fluidizing gas through it, then pouring the mould, then restoring the passage of fluidizing gas through the refractory material to fluidize the latter, and finally removing the poured mould from the bed.

2. A method of making a shell-moulded article comprising the steps of first passing a fluidizing gas through a bed of refractory granular material to fluidize the latter, effecting a reduction, relative to the permeability of the fluidized bed, of the permeability of the shell mould having a downgate to a fluidizing gas by treating the shell mould by enveloping it with a substance that is impervious to the fluidizing gas and which is substantially destroyed during the pouring of the mould, after which the mould is immersed in the bed to a depth such that, on collapse of the fluidized bed, the mould is supported by the refractory material with said downgate clear of the surface of the material, and then collapsing the bed by ceasing to pass fluidizing gas through it, then pouring the mould, then restoring the passage of fluidizing gas through the refractory material to fluidize the latter, and finally removing the poured mould from the bed.

3. A method as claimed in claim 2 in which the mould is enveloped in a sheet, bag or container of the substance.

4. A method as claimed in claim 2 in which the external surface of the mould is coated with the substance.

5. A method as claimed in claim 3 in which the substance is a plastics material.

6. A method as claimed in claim 5 in which the plastics material is polyvinyl chloride.

7. A method as claimed in claim 4 in which the substances is paraffin wax.

8. A method as claimed in claim 1 in which the reduction is effected by coating the external surface of the shell mould with a substance which decreases the permeability of the mould to said value without affecting, substantially, the permeability of the mould to gases emitted during pouring.

9. A process as claimed in claim 8 in which the substance is bentonite.

10. A process as claimed in claim 1 in which reduction of permeability is effected by closure of the or each downgate prior to immersion of the shell mould in the fluidized bed followed by opening of the or each downgate prior to pouring the mould.

11. A process as claimed in claim 1 in which the refractory granular material has a relatively high thermal conductivity and has a permeability to gases evolved during pouring of a mould of a value greatly in excess of the permeability of the mould.

12. A process as claimed in claim 1 in which the bed is vibrated after collapsing the fluidized bed and before the mould is poured in order to compact the refractory material round the mould. 

2. A method of making a shell-moulded article comprising the steps of first passing a fluidizing gas through a bed of refractory granular material to fluidize the latter, effecting a reduction, relative to the permeability of the fluidized bed, of the permeability of the shell mould having a downgate to a fluidizing gas by treating the shell mould by enveloping it with a substance that is impervious to the fluidizing gas and which is substantially destroyed during the pouring of the mould, after which the moulD is immersed in the bed to a depth such that, on collapse of the fluidized bed, the mould is supported by the refractory material with said downgate clear of the surface of the material, and then collapsing the bed by ceasing to pass fluidizing gas through it, then pouring the mould, then restoring the passage of fluidizing gas through the refractory material to fluidize the latter, and finally removing the poured mould from the bed.
 3. A method as claimed in claim 2 in which the mould is enveloped in a sheet, bag or container of the substance.
 4. A method as claimed in claim 2 in which the external surface of the mould is coated with the substance.
 5. A method as claimed in claim 3 in which the substance is a plastics material.
 6. A method as claimed in claim 5 in which the plastics material is polyvinyl chloride.
 7. A method as claimed in claim 4 in which the substances is paraffin wax.
 8. A method as claimed in claim 1 in which the reduction is effected by coating the external surface of the shell mould with a substance which decreases the permeability of the mould to said value without affecting, substantially, the permeability of the mould to gases emitted during pouring.
 9. A process as claimed in claim 8 in which the substance is bentonite.
 10. A process as claimed in claim 1 in which reduction of permeability is effected by closure of the or each downgate prior to immersion of the shell mould in the fluidized bed followed by opening of the or each downgate prior to pouring the mould.
 11. A process as claimed in claim 1 in which the refractory granular material has a relatively high thermal conductivity and has a permeability to gases evolved during pouring of a mould of a value greatly in excess of the permeability of the mould.
 12. A process as claimed in claim 1 in which the bed is vibrated after collapsing the fluidized bed and before the mould is poured in order to compact the refractory material round the mould. 